The present invention relates to a floating wind turbine and a method for controlling the vertical extent of the wind turbine. More particularly it relates to a floating windmill for electric power production, the windmill comprises a hull attached to a supporting column for a wind turbine which is arranged to be put into rotation by wind force acting on rotor blades, the wind turbine is connected to a buoyancy device by means of a coupling device, the buoyancy device is arranged encircling at least a portion of the hull, and the buoyancy device is arranged to be able to support at least a portion of the mass of the wind turbine.
The purpose of the present invention is to provide an apparatus and a method which will enable simplifying transport and erecting of a floating wind turbine at sea by heeling the wind turbine relative to the buoyancy device which is floating at the sea surface, and thereby the wind turbine may be laid down and erected in a controlled way, preferably without external lifting devices, for the purpose of reducing the draft of the hull and the height of the supporting column.
In the light of the continuously increasing demand for environmentally friendly and renewable energy, it is desirable to harvest the resources from wind energy, particularly at open sea where occurrences and available areas are particularly large and installations for energy production moreover are of relatively little nuisance and without encroachments on nature. The need and the market for wind turbines for electric power production at sea have therefore increased, and various solutions for fixed and floating installations have been launched.
Prior art for installation of wind turbines at sea can principally be divided into two categories; installations fixed to the sea floor and floating installations, suited for shallow and deep areas of the ocean, respectively.
Various forms of foundations fixed to the sea floor have been launched, where some are already well established on the market, while others are on a conceptual level.
Most widespread so far are steel jackets fixed to the sea floor by means of piles. This solution is well suited for minor depths of the ocean. This solution and similar solutions have in common that installation of supporting column and wind turbine, and also later maintenance and repair, must take place at open sea by means of large and costly crane vessels, where weather and wave conditions may be a complicating and limiting factor.
Other kinds of constructions fixed to the sea floor are based on solutions with a steel or concrete foundation having buoyancy and stability, and the installations may thereby be completed at shore before they are towed floating to the field and put down on the sea floor for mainly permanent installation. Future maintenance and repair must therefore be carried out in situ.
Still others have a floating hull with support legs which can be lowered down and placed on the sea floor in such a way that the hull can be raised above the sea surface. This solution is intended for a non-permanent installation, and enables relatively simple reversing and return to shore for maintenance and repair.
Prior art for floating installations mainly comprises isolated supporting columns floating upright in the sea, and stable hulls with one or more supporting columns.
Embodiments with floating columns may be assembled in calm waters, but due to the draft of the construction in order to achieve satisfactory stability, relatively large water depths are required both for the assembly and the transport route to the anchoring field, and the embodiment is therefore unsuitable for assembling and later maintenance at shore.
Known embodiments of floating hulls with one or more supporting columns for wind turbines mounted thereon have the advantage of being able to be both completed and later maintained and repaired at shore, but are comparatively more expensive to build due to the required mass and size of the hull in order to achieve satisfactory stability.
The publication NO 330281 B1 discloses a floating wind turbine, including a rotor, an upper column attached to the rotor, and a stabilizer tank arranged between the upper column and a lower column, where the stabilizer tank has its center of buoyancy eccentrically arranged relative to a longitudinal center axis to the upper and lower column.
Further floating wind turbines are known from publication WO 2011051804 A1, U.S. 2011179986 A1 and DE 20100588 U1.
The present invention has for its object to remedy or reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.
The object is achieved through features specified in the description below and in the appended claims.